矩阵操作中的nonzero()

老规矩，贴源码解释

    def nonzero(self):
        """nonzero indices

        Returns a tuple of arrays (row,col) containing the indices
        of the non-zero elements of the matrix.
        返回包含矩阵的非零元素索引的数组(行、列)的元组

        Examples
        --------
        >>> from scipy.sparse import csr_matrix
        >>> A = csr_matrix([[1,2,0],[0,0,3],[4,0,5]])
        >>> A.nonzero()
        (array([0, 0, 1, 2, 2]), array([0, 1, 2, 0, 2]))
        左边的array代表行索引，右边的array代表列索引，(行索引，列索引)代表一个矩阵元素

        """

        # convert to COOrdinate format
        A = self.tocoo()
        nz_mask = A.data != 0
        return (A.row[nz_mask], A.col[nz_mask])